The print part is a generic function in R. A generic function is a generalized concept, for example, printing results is a generic concept and the corresponding generic function is print. This function works differently based on the class of the objects. The object x here has the class robustSummary. As a result, whenever you call the object itself into the R console, it actually calls the print.robustSummary() function.

However, before calling the print.robustSummary() function, you must define it and specify the behavior of the function; for example, it should know what to print, what should be the format, and so on.